1. Field of the Invention
The area of the present invention is that of audio mixers and, more particularly, the complexity required of a network to weight and add together multiple analog audio inputs.
2. Background Art
The evolution of computer technology has produced computer systems that are capable of simultaneously processing a variety of audio and video inputs. Such systems have become known in the art as “multimedia” computers. In a multimedia environment, audio signals from various input sources are typically summed and communicated to common output devices. These sources may include sound generated by the computer, internal CDROMs, external CDROMs, internal microphones, telecommunications DAAs, television audio, broadcast radio, and the like.
The prior art applied to this problem has been one of arranging a means to vary the level of each audio signal and then computing the algebraic sum of the various levels using analog circuitry well known in the art. This technique is straightforward and capable of excellent results. Typically, one level adjusting circuit is used for each input channel. A stereo input source (e.g., CDROM) will therefore require two channels.
While the prior art has used any number of methods to vary the level of an analog signal, Multimedia systems typically use some form of a multiplying D/A converter which allows discrete adjustment of the level, rather than the continuous adjustment of a volume control potentiometer. The discrete control uses a minimum gain variation chosen to be inaudible, usually agreed to be less than 1–2 dB.
In typical analog systems, each channel needing adjustment requires an operational amplifier “gain block”, a resistor network to set the amount of gain, and an array of analog switches to select which tap of the resistor network will be used. A typical configuration will have the gain vary from 0 to +22.5 dB in +1.5 dB steps, requiring 16 analog switches. Given the ever increasing demands for integration of multimedia components, it is easy to justify mixing 6 stereo sources. Using the techniques of the prior art this will require 12 amplifiers, 192 resistors, and 192 analog switches just to implement the level adjusting portion of the circuit.
In general, personal computer functions supporting such multimedia audio capability will be supported within a single Application Specific Integrated Circuit (ASIC). In general, the level adjusting and mixing scheme needed in an audio ASIC is used to maximize the signal to quantization noise ratio in an A/D converter. Consequently, the level adjusting scheme must typically increase the signal amplitude and add the resulting amplified analog signals together. When attempting to apply the means of the prior art to an equivalent circuit in an ASIC, a large amount of expensive silicon die area is consumed. Accordingly, there exists a need for a reduced complexity audio mixing apparatus which provides a capability of mixing a number of input sources with a minimum number of analog components.